Chiral supramolecular assemblies from achiral and chiral molecules investigated by second harmonic generation at the air-water interface

Abstract

We report the use of Second Harmonic Generation (SHG) to investigate at the air-water interface molecular films of PL2(-) molecules, a chiral binaphthyl derivative. Under the compression of the monolayer film in a Langmuir trough, large fluctuations of the SHG intensity are observed. From the expressions of the SHG intensity with polarization control of the input fundamental and output harmonic beams, it appears that the intrinsic chirality of the PL2(-) molecule can be disentangled from the supramolecular chirality arising from PL2(-) molecular aggregates. With a careful polarization control of the input and output polarization configurations of the SHG optical set-up, it is then shown that the dominant origin of the observed SHG intensity fluctuations is the formation of PL2(-) molecular aggregates at the air-water interface. The proposed strategy is also suitable for the analysis of the fluctuations in the SHG intensity arising from molecular films at the air-water interface formed from achiral molecular compounds.